1. The Field Of The Invention
The present invention relates generally to X-ray tube technology. More specifically, this invention pertains to a new configuration for a low power X-ray source for an X-ray fluorescence instrument having an air-cooled and metal-ceramic design which provides for a higher flux of X-rays as compared with X-ray tubes of similar power input. Most advantageously, the configuration of the cathode assembly and the anode assembly is such that a small nose at the end-window is provided, thereby enabling the X-ray source to be close to a sample being irradiated.
2. The State Of The Art
In many typical state of the art X-ray tubes, a cathode assembly and an anode assembly are vacuum sealed in a glass envelope. Electrons are generated by at least one cathode filament in the cathode assembly. These electrons are accelerated toward the anode assembly by a high voltage electrical field. The high energy electrons generate X-rays upon impact with the anode assembly. An unavoidable by-product of this process is the generation of substantial amounts of heat. It is important to the life of the X-ray tube to dissipate the heat as efficiently as possible.
The X-ray tube described above is mounted within a housing for protecting the surrounding environment from unwanted X-rays. A state of the art method for cooling the X-ray tube is to fill the housing with oil. The oil not only provides electrical insulation, but it also absorbs the heat generated by the anode assembly. The requirement of an oil pump and hoses also results in lower system reliability, the possibility of leaks and fire, as well as extra cost. Oil cooling also makes repair and maintenance of the X-ray tube more difficult.
Alternatives have been developed to use in place of oil. For example, although sulfur hexaflouride (SF6) is preferable to oil for various reasons, it is expensive, difficult to handle safely, and it can reduce a high voltage standoff capability when it leaks.
An important feature of an X-ray tube utilized in X-ray fluorescence (XRF) is that the X-ray source be as close as possible to a subject or sample being irradiated. The result of X-rays being absorbed by the sample is that it fluoresces. A detector of fluorescent energy is then disposed near to the sample at a desired angle relative to the sample and the X-ray source. The desired angle typically enables the maximum amount of fluorescent energy to be received by the fluorescent energy detector.
X-ray tubes which are utilized in X-ray fluorescence instruments are typically characterized as being one of three different X-ray tube configurations. These X-ray tube configurations are known as a transmission tube design having an end-window from which X-ray energy is directed toward a sample, and a side-window configuration.
There are inherent drawbacks to each X-ray tube design which hinder their performance in XRF instruments. The components of the transmission tube 10 which are of relevance to the present invention are illustrated in general in FIG. 1. FIG. 1 shows that a housing 12 surrounds a cathode assembly 14. The cathode assembly 14 is centered behind an anode/window combination 16. In this way, a high voltage field developed between the cathode assembly 14 and the anode/window 16 causes electrons 18 emitted from a filament (not shown) in the cathode assembly 14 to flow directly toward the anode/window 16. For example, the anode/window 16 can be coated with an anode-type material. The electron flux 18 striking the anode/window 16 causes the generation of X-rays 20. The usable X-rays 21 continue out through the anode/window 16. Accordingly, instead of electrons 18 striking an anode and the resulting X-rays 20 being deflected therefrom at an angle, the usable X-rays 21 continue on in the same direction as the original flow of electrons 18 from the cathode assembly 14.
There are several disadvantages to this design. First, there are reliability drawbacks. The high voltage stability of a transmission tube is generally not as good as from a side-window X-ray tube design. The anode/window is also constructed differently because of the substantial amount of heat which is generated. This heat imposes a limit on how thin the anode/window can be constructed. Disadvantageously, the X-rays produced on the surface of the anode are substantially attenuated on passing through the entire thickness of the anode window. Consequently, the X-ray emissions are not as strong as they could be.
The end-window tube design has inherent design drawbacks which prevent it from being more useful in an X-ray fluorescence detector. Specifically, the size of the X-ray tube nose interferes with optimum detector placement.
Unfortunately, the side-window X-ray tube also has serious drawbacks which typically prohibit or hinder its application in XRF instruments. These drawbacks stem from the fact that the sample-to-target distance is necessarily large. The distance is large because as shown in the cross section view of an X-ray tube 22 provided in FIG. 2, the X-ray tube itself interferes with the detection of fluorescent energy 24 because a fluorescent energy detector 26 can not be placed in optimal locations. In other words, the sidewall 28 of the X-ray tube 22 absorbs much of the fluorescent energy 24 which would otherwise be detected at the optimal detection angle. However, moving the side-window X-ray tube further from the sample simply decreases the available X-ray flux at the sample. The available X-ray flux is already inherently small due to the large distance from the target 31 to the sample 30 in the side-window tube.
It is also worth noting that the state of the art X-ray tubes suffer from a relatively short filament life, poor stability and high tube electrical leakage.
Therefore, it would be an advantage over the state of the art to provide an X-ray tube which enabled greater X-ray emissions to reach the sample, and then permit a fluorescent energy detector to be located at an optimal angle of deflection, and with a minimal target-to-sample distance and superior detector-sample coupling.
There are other features in state of the art X-ray tubes, both end-window and side-window designs, which are also disadvantageous. For example, glass is commonly utilized as a high voltage insulator. But the glass is subject to fracture. The glass also results in a less repeatable manufacturing process which increases costs. Glass also prohibits higher temperature tube processing which facilitates the elimination of additional gas from the vacuum envelope in the X-ray tube.
It would be another advantage over the state of the art to replace glass with a more rugged high voltage insulator. It would be further advantageous if the high voltage insulator would then permit higher temperature processing to thereby enhance tube processing to obtain a better vacuum and thus a cleaner X-ray tube. It would also be advantageous to provide an X-ray tube with better heat transfer characteristics which would enable the anode to operate at a lower temperature, and thus extend the operational life of the X-ray tube.